1. Field of the Invention
The present invention relates to a toroidal continuously variable transmission and a method of manufacturing the same. More specifically, the present invention relates to a toroidal continuously variable transmission designed for use in a vehicle, such as an automobile, and a method for manufacturing the same.
2. Description of the Related Art
As a toroidal continuously variable transmission, the one shown in FIG. 1 has been conventionally known.
Inside a housing (not shown), an input disk 1 and an output disk 2 are coaxially arranged in such a manner that they face each other. An input shaft 3 extends through the axis of the toroidal transmission including the input and output disks 1 and 2. A loading cam 4 is disposed at one end of the input shaft 3. The loading cam 4 is configured to transmit the driving force (torque) of the input shaft 3 to the input disk 1 through a cam roller 5.
The input and output disks 1 and 2 are substantially similar in shape and arranged to be symmetric. The facing surfaces of these disks are toroidal surfaces and jointly form substantial semicircles when viewed in a section taken axially. A pair of power roller bearings 6 and 7, which are in contact with the input and output disks 1 and 2, are inserted into toroidal cavities defined by the toroidal surfaces of the input and output disks 1 and 2.
Power roller bearing 6 includes a power roller 6a which rolls on the toroidal surfaces of the input and output disks 1 and 2 (and which corresponds to an inner ring of the power roller bearing 6), an outer ring 6b and a plurality of rolling members (steel balls) 6c. The other power roller bearing, i.e., power roller bearing 7, includes a power roller 7a which rolls on the toroidal surfaces of the input and output disks 1 and 2 (and which corresponds to an inner ring of the power roller bearing 7), an outer ring 7b and a plurality of rolling members (steel balls) 7c. 
As can be seen from the above, the power roller 6a functions as the inner ring that is a structural element of the power roller bearing 6, and the power roller 7a functions as the inner ring that is a structural element of the power roller bearing 7. The power roller 6a is rotatably coupled to a trunnion 10 by means of a pivot 8, the outer ring 6b and the rolling members 6c, and at the same time can be inclined in such a manner that a pivot axis O, which is the center of the toroidal surfaces of the input and output disks 1 and 2, is the center of the inclining movement.
Likewise, power roller 7a is rotatably coupled to a trunnion 11 by means of a pivot 9, outer ring 7b and rolling members (steel balls) 7c, and at the same time can be inclined in such a manner that a pivot axis O, which is the center of the toroidal surfaces of the input and output disks 1 and 2, is the center of the inclining movement. A lubricant having a great viscosity or friction resistance is supplied onto the surfaces of the input and output disks 1 and 2 and those of the power rollers 6a and 7a. A driving force exerted on the input disk 1 is transmitted to the output disk 2 through a lubricant film and the power rollers 6a and 7a. 
The input and output disks 1 and 2 are independent of the input shaft 3 by means of a needle 12 interposed between them (that is, the input and output disks 1 and 2 move without reference to the rotating shaft 3). The output disk 2 is connected to an output shaft 14 which is parallel to the input shaft 3 and is rotatably supported by a housing (not shown) by means of angular members 13.
In the toroidal continuously variable transmission 20, the driving force of the input shaft 3 is transmitted to the loading cam 4. When the driving force, thus transmitted, rotates the loading cam 4, the rotation is transmitted to the input disk 1 through the cam roller 5, thereby rotating the input disk 1. The driving force based on the rotation of the input disk 1 is transmitted to the output disk 2 by way of the power roller 6a and the power roller 7a. As a result, the output disk 2 rotates together with the output shaft 14.
At the time of transmission, the trunnions 10 and 11 are moved a little toward the pivot axes O. By moving the trunnions 10 and 11 toward the axes, the intersection between the axis of rotation of the power rollers 6a and 7a and the axis of the input and output disks 1 and 2 is shifted slightly from the original position. As a result, the circumferential velocity of the rotation of the power rollers 6a and 7a and that of the rotation of the input disk 1 become off balance, and part of the torque of the input disk 1 serves to rotate the power rollers 6a and 7a around the pivot axes O. As a result, the power rollers 6a and 7a slide and incline on the curved surfaces of the input and output disks 1 and 2.
When the toroidal continuously variable transmission is driven, high contract pressure tends to act between the input and output disks 1 and 2 on one hand and the power rollers 6a and 7a on the other hand, and the maximal value of the contact pressure is as high as 4 Gpa or thereabouts. In addition to this problem, the bearing surfaces of the power rollers 6a and 7a are also exerted with high pressure per unit area, and may peel off in a short time.
Under the circumstances, peeling or damage to the traction portions of the toroidal surfaces of the input and output disks 1 and 2, those of the power rollers 6a and 7a, and those of the power rollers 6a and 7a must be prevented for increasing reliability. In addition, since the input and output disks 1 and 2 and the power rollers 6a and 7a must repeatedly undergo bending stress, fatigue cracking tend to occur. The input and output disks 1 and 2 and the power rollers 6a and 7a must be improved so that they hardly suffer fatigue and can withstand long use against a bending stress.
Jpn. Pat. Appln. KOKAT Publication No. 7-71555 is intended to provide a toroidal continuously variable transmission wherein the traction surfaces of input and output disks and those of power rollers have an improved rolling-resistant life, and wherein the input and output disks and the power rollers have an improved anti-fatigue cracking life. To attain this object, the disks and the power rollers are subjected to carbonitriding and grinding. Alternatively, they are subjected to carbonitriding and grinding, and are further processed in such a manner that at least either the disks or the power rollers have a hardened layer whose effective depth is not less than 2.0 mm and not more than 4.0 mm.
According to Jpn. Pat. Appln. KOKAI Publication No. 2001-82566, at least one of an input disk, an output disk and a power roller is formed of medium carbon steel having a carbon content of 0.3% to 0.5%, and is subjected to carburization or carbonitriding processing and further to hardening and annealing so that the hardness of the core portion is not less than HRC35. By this processing, an improved life against fatigue is attained.
However, the disks and power rollers are improved by carburization executed at a temperature in the range of 920 to 960xc2x0 C. or by carbonitriding processing executed at a temperature in the range of 930 to 950xc2x0 C., and this process is continued 20 to 30 hours. After the heat treatment, therefore, a large-sized carbide or carbonitride is undesirably formed in the metal structure of the disks and power rollers. Since this carbide or carbonitride degrades the fatigue strength of the disks and power rollers, the formation of such a substance must be prevented. In the case of a traction drive such as a toroidal continuously variable transmission, the traction force exerted on the rolling surfaces gives rise to a tangential force. If a carbide or carbonitride is present in a crystalline grain boundary, a stress tends to concentrate at such a position, causing cracks. As a result, peeling or fracture may occur.
The present invention has been conceived to solve the above problems, and is intended to provide a toroidal continuously variable transmission which has an improved life against fatigue by producing minute particles of carbide or carbonitride and which reduces the heat treatment cost by shortening a carburization or carbonitriding processing time. The present invention is also intended to provide a method for manufacturing the toroidal continuously variable transmission.
To achieve this object, the toroidal continuously variable transmission system of the present invention comprises: an input disk and an output disk which have traction surfaces defined between a small-diameter portion and a large-diameter portion and having an arc-shaped concave section and which are coaxially arranged such that the traction surfaces face each other; and power roller which are frictionally engagement with the traction surfaces of the input and output disks and transmit a driving force, characterized in that at least one of the input disk, the output disk and the power rollers is formed of an alloy steel which contains 0.15 to 2.0% by weight of Mo, which is subjected first to carburization or carbonitriding processing executed at a temperature not lower than 1,000xc2x0 C. and then to hardening and tempering, and which has a surface carbon content of 0.8 to 1.4%.
The toroidal continuously variable transmission of the present invention is characterized in that a surface nitrogen concentration is in the range of 0.04 to 0.3%
The invention is characterized in that at least one of the input disk, the output disk, and the power roller and outer ring of a power roller bearing has a carbon content equal or more than 0.3% and equal or less than 0.5%.
The method of the present invention is applied to the manufacture of a toroidal continuously variable transmission comprising: an input disk and an output disk which have traction surfaces defined between a small-diameter portion and a large-diameter portion and having an arc-shaped concave section and which are coaxially arranged such that the traction surfaces face each other; and power roller which are frictionally engagement with the traction surfaces of the input and output disks and transmit a driving force, characterized in that the method comprises: forming at least one of the input disk, the output disk and the power rollers by use of an alloy steel which contains 0.15 to 2.0% by weight of Mo; executing carburization or carbonitriding processing at a temperature not lower than 1,000xc2x0 C.; then executing hardening and tempering, such that a surface carbon content is within the range of 0.8 to 1.4%.
The invention described above is characterized in that a surface nitrogen concentration is in the range of 0.04% to 0.3%.
The invention described above is characterized in that at least one of the input disk, the output disk, and the power roller and outer ring of a power roller bearing has a carbon content equal or more than 0.3% and equal or less than 0.5%.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.